This invention relates to a process and apparatus for determining characteristics of coal by its initial rate of desorption of methane gas under specific test conditions.
As part of its structure coal contains pores and fissures of a few Angstroms to several millimeters in size. Pores function as gas reservoirs whereas fissures act as transport arteries. The initial rate of desorption of gas from coal is very great and depends on the degree of fissuration. As the gas continues to flow out from the very fine pores (0.01 .mu.m) and fissures, the rate of desorption decreases and the emission follows the laws of diffusion. It has been shown that coal friability is associated with the presence of a large number of fissures 5 .mu.m in thickness. A useful indication of fissuration is the .DELTA.P index, a measure of the initial rate of desorption which depends on micro-fissuration and perhaps also on the size and distribution of pores in the coal. The .DELTA.P index is described in Ettinger et al "Comparative Sorption of Carbon Dioxide and Methane", Fuel 45:351; 1966, and has been used in France, Belgium and other countries to obtain a measure of the rate of desorption of methane from coal and as an indirect means of classifying coal structure. Studies by CERCHAR of France have shown that .DELTA.P indices of various sizes of fragments stay nearly constant as long as the size of the coal particles is greater than the distance between fissures. The .DELTA.P index increases if particle sizes are less than this distance because comminution develops more new surfaces than were originally available, i.e, the particle size for which the .DELTA.P index begins to increase abruptly corresponds closely to the spacing of the fissures.
The .DELTA.P indices are empirical values which depend on the test being carried out in accordance with standard procedures and using equipment of standard size. As such, the .DELTA.P measurement provides a means of comparing the rates at which gas can be released from different coals. The movement of gas out of coal particles probably occurs as a two-stage process, the gas first desorbing and diffusing through the fine pore structure and then flowing more rapidly through any fissures present.
The .DELTA.P index can provide very useful information including:
(a) a prediction of the amount of fines during regular coal-mining operation and at early development stages;
(b) a measure of the degree of coal oxidation before beneficiation and during stockpiling and storage; and
(c) a measure of the susceptability to gas outbursts in underground coal mines.
The .DELTA.P measurement proceeds essentially by saturating a coal sample in a sample cup with methane gas at one atmosphere pressure until equilibrium adsorption of the methane into the surfaces of the coal is attained. The sample cup volume is then opened into an additional initially evacuated manifold volume. This causes an immediate drop in the methane pressure due to the increased total system volume. The reduced pressure results in a corresponding decrease in the equilibrium mass of methane that may remain adsorbed in the coal surfaces. The excess methane thus begins to desorb into the system volume at a rate dictated by the time constant of the sorption/desorption mechanism. The initial resulting rate of pressure rise is the measured quantity used to characterize the coal in the .DELTA.P index measurement.
The exact sample cup and manifold volumes are specifics of the standarized experiment as is the initial gas pressure of one atmosphere. The .DELTA.P index refers to the initial rise in gas pressure due to desorption over a specific interval of 50 seconds. In order that an exact coal sample density not be required, the pressure drop due to the initial opening to the manifold volume is also a recorded parameter.
The existing systems for measuring .DELTA.P index have utilized a mercury manometer for measuring pressure and have utilized manual valves for switching the flows in the manifold. The manometer suffers from transient effects and cannot be reliably read until about 10 seconds after opening of the valves. This causes the determination of a .DELTA.P index to be a very slow and laborious process.
It is the object of the present invention to provide a simpler, faster and more accurate system for determining the .DELTA.P index.